So, here's what I've learned after playing this game for like 4 hours.
there's a 30 day limit per play though, after the 30th day, you get the bad end.
To level up you have to do the H scenes, fighting monsters actually lower your exp.
every.... lets say 10 levels, you unlock the next part of a H scene. you can find out which scene has more than 1 part in the basement of the starting house.
You cannot collect all H scenes in one go, even if you could, you cant get the 2nd and 3rd h scenes involving teleporters in the 2nd to last village because it freezes for some reason.
H scenes require HP, more HP is required for the next level of H scene.
If you don't get the real ending (which I still haven't) after the 30 days you start a new game+, the difference is you keep your level exp. That means you keep your last games HP and MP, hopefully beating the starting levels faster.
Costume unlock methods (that I know of from my first play through):
Brown sweater dress, purchasable in the starting town. for like 100g you get the shorter variant when doing the bar scene for the first time
Blue negligee, obtainable during the 2nd guild quest
Swimsuit (this needs to be equipped to enter beach south of starting town) To get the swim suit you need to complete the 3rd guild quest (its the one where you have to enter a girls dream and rescue her from a succubus demon thing)
Arena Uniform, you guessed it, you get this from participating in the arena for the 1st time
Pink negligee, obtainable in the last village upon entry. I found out its some sort of slum area and everyone you talk to initiates a H scene (free exp i suppose)
Virgin Killer Sweater: Unlocked after accepting the bar job after leveling up h events to max
I'll update this when I discover more